The invention relates to a printing machine having at least one printing unit whose image cylinder is driven by friction by an element to which the image is transferred.
In such printing machines, it has been shown that, as a result of a change in the images or the printing substrates, a change in the rotational speed of the image cylinder can occur. Changes which affect the rotational speed are, in relation to the images, the toner application, that is to say, toner thickness, toner distribution and image width and, in relation to the printing substrates, their thickness, width and possibly also other properties of the printing substrate.
This leads to register inaccuracies when the aforementioned variables change from one printed page to the next, since, as a result of the change in the rotational speed of an image cylinder, both the assignment of an image to the printing substrate and the assignment of the color separations to one another become faulty. In order to counteract register faults, in the printing machines of known type, register marks are printed and evaluated in order, for example, to control digital image production appropriately. However, this type of problem solution requires time and, as a result, it is in particular not possible to change seamlessly from one printed page to another without interrupting the running of the machine. Such a seamless transition is opposed by the outlay on time for test prints and evaluation, and by the fact that in the event of changes without interrupting the printing operation, printed pages, which have differences with regard to the images and/or the printing substrates, affect each other. It is particularly important to rule out the influence of such changes if the pages printed one after another have differences in terms of content and paper.
The invention is, therefore, based on the object of developing a printing machine of the type mentioned at the beginning in such a way that the influence of a change in image contents and/or variables is kept as small as possible. As an additional object, the intention is also for the influence of changes in the printing substrates to be kept as small as possible.
According to the invention, the object is achieved by the width of image cylinder and element being so much wider than the maximum width of the image to be transferred that the rotational speed of the image cylinder remains essentially constant in the event of a change in the image.
The invention is based on the finding that the influence of a change in the aforementioned variables on the rotational speed of the image cylinder driven by friction has an effect only within specific ranges of the image and/or printing-substrate width in its relationship to the width of the image cylinder and of the element driving the image cylinder. In this case, there is a range, once a minimum difference has been exceeded, by which the image cylinder and the element driving the image cylinder are wider than the image width and in which changes in the abovementioned variables no longer exert any significant influence on the rotational speed of the image cylinder. After exceeding this minimum difference, this range is not limited. The invention is based on utilizing this range. However, this range cannot be given in concrete dimensions, since the actual position depends on the configuration of a machine type. It is, therefore, necessary for this range to be determined for each machine type, which can be done empirically, for example. In a corresponding way, there is also such a range in relation to the printing substrates, the elements carrying the printing substrate likewise having to be wider by a minimum dimension than the printing substrates.
The invention makes a printing machine available which, with regard to its register accuracy in the event of changes in the abovementioned variables, exhibits a high constancy. As a result, no corrections, or significantly fewer corrections, to the register are needed in the event of a change in the aforementioned variables. In this way, the lost time to take changes of the aforementioned variables in account is reduced considerably or to zero. It is often the case that, as a result, a printing machine is able to print one printed page after the other printed page, different from the last printed page, seamlessly. The invention, therefore, increases the performance and the register accuracy of a printing machine, and a high degree of economy can even be achieved during the continuous printing of different printed pages or in the case of relatively small editions.
Depending on the configuration of the actual printing machine, the element, which drives the image cylinder by friction, may be an element of a very wide range of types. It can likewise be a cylinder or a web guided by rolls. It can even be the carrier for printing substrates or an image transfer element of any desired design, which transports the images onward to a printing-substrate carrier.
The dimensioning of the width of the image cylinder and that of the element or elements driving the latter depends on the image width if only pure image transfers take place in the drive train for the image cylinder. This dimensioning is based on the printing-substrate width and, as a result, will become greater, by about the width of the print-free edge, if images are transferred to printing substrates in this drive train.
One embodiment of the invention, therefore, provides for the element which drives the image cylinder to be the carrier which carries the printing substrates during the printing operation, and for the width of image cylinder and element to be at least so much wider than the maximum width of the printing substrate that the rotational speed of the image cylinder remains essentially constant in the event of a change in the printing substrates. In this case, instead of the width of the image, the width of the printing substrate has an effect, but the printing substrate either being equally as wide as the image or wider by a mostly relatively narrow image-free edge.
In a different configuration of the printing machine, the element is an image transfer cylinder. If the drive is provided directly via the image transfer cylinder or cylinders, then only the image width is critical for dimensioning the width of the image cylinder and of the image transfer cylinder. If, however, provision is made for the image transfer cylinder to be driven by the carrier for the printing substrates, then provision must also be made here for the width of image cylinder, image transfer cylinder and carrier to be at least so much wider than the maximum width of the printing substrate that the rotational speed of the image cylinder remains essentially constant in the event of a change in the aforementioned variables. In the case of such transfers of force, at least when a printing substrate is led between elements, one of the elements must have a certain elasticity which ensures the transfer of force even without the inclusion of a printing substrate.
Furthermore, provision is preferably made, on the side of the carrier, which is opposite the image transfer, to arrange an impression cylinder, which likewise has at least the abovementioned width. In this case, this is the width, which is dimensioned in accordance with the maximum width of the printing substrate. With regard to the carrier, provision may be made for this to be driven via a drive roller.
The measure according to the invention is preferably used in printing machines in which the image is a toner image, since this has a thickness, a profile and a size which, in normal operation of the machine, very often change from one printed page to the next. These are mostly printing machines, which are equipped with equipment for digital image production, as a rule, equipment for digital electrostatic image production, equipment for toner application and for toner removal then additionally being present. In the case of such printing machines, each image is produced anew, so that a machine of this type is preferably used in a sector in which each printed page is different from the preceding and the following one.